The efficiency of a power amplifier, such as used to amplify wireless signals for transmission, largely depends on the load impedance of the amplifier. For example, a small load impedance may result in a large output power at the expense of a large output current, which translates into a low efficiency. Contrastingly, a large load impedance may result in less output power and current consumption, and thus have a higher efficiency. Because the amplifier load impedance directly affects the amplifier efficiency, it is desirable to be able to control the amplifier load impedance based on a desired performance. Further, such load impedance control desirably facilitates the ability to more closely match the load impedance of the amplifier to that of an element connected to the amplifier, e.g., an antenna.
One option varies the load impedance using biased pin diodes and capacitors. While a pin diode is an ideal RF switch, pin diodes cannot be integrated with a CMOS chip. Thus, the pin diode option is not desirable for on-chip power amplifiers.
Another option implements load impedance switching using variable LC networks that may be integrated on the chip with the power amplifier. Such LC networks, however, require bulky inductor coils, and therefore, undesirably increase the size of the chip.
Thus, there remains a need for small, on-chip options for varying the load impedance of an on-chip power amplifier.